It may occur that waste water contains rather big objects, e.g. plastic sacks containing night-soil, which with conventional, self-hoisting rinsing screens are difficult or impossible to handle. Another problem is that the screenings becomes tangled into balls, which may be so big that they can not be conveyed by the self-hoisting rinsing screens. The only possibility of solving these problems hitherto has been by manual operations, i.e it has been necessary to remove the objects by hand. As different types of rinsing screens, e.g. chain screens, step screens and the like operate fully automatically for long times such bigger objects may cause clogging and other types of breakdowns.
Another problem at self-hoisting rinsing screens of the type consisting of fixed and movable rods, having step-shaped edges facing the flow direction of the water, whereby the movable rods are interconnected to at least one pack, is that the movable rods are raised to a level above the lowermost step, which means that the lower part of the screen during this upward movement has a double slot width between the rods resulting in an inferior separation efficiency.